Process for online power cut out of an aluminum reduction cell

ABSTRACT

A process for online cut out of an aluminum reduction cell (pot) including the steps of leveling of the side beams of the reduction cell to around 150 mm; punching holes on both sides of the pot to ensure that the bath does not get overflow and ensure the dipping of the anodes through these punch holes; blowing air thoroughly all around the insulated shorted joints of all the risers to clean the joints from dust and large chunks of bath; applying lubricating oil to the shorted joint bolts to conform proper movement of nuts; breaking the side crust of all the four corner anodes; tapping the bath with at least one ladle; lowering the anodes such that there is no liquid overflow over the deck plate onto the catwalk; lowering of the beam by around 70 mm to 100 mm until the voltage falls below around 1.0 V-1.5 V at around 320 kA; loosening all riser bolts of the shorted joint of all the five risers using a pneumatic wrench after confirming that the anodes are dipped into the metal; removing insulated insert plates from the shorted anode riser joints and retightening all the five riser bolts using a pneumatic wrench and then taking the pot offline.

BACKGROUND

1. Field of the Invention

The present invention relates generally to a process for an online power cut out of aluminum reduction cell and more particularly a process for isolating (cut off) a pot (aluminum reduction cell) online during continuous production without affecting or influencing other cells connected in series.

2. Description of the Prior Art

Metal aluminum is produced industrially by igneous electrolysis, i.e. by means of electrolysis of alumina in solution in a molten cryolite bath, referred to as an electrolyte bath according to the well-known GAMI technology. The electrolyte bath is contained in pots, referred to as “electrolytic pots”, having a steel shell which is lined internally with refractory and/or insulating materials with a cathode assembly located at the base of the pot. Anodes made of carbonaceous materials are partially immersed in the electrolyte bath. The assembly formed by an electrolytic pot, its anode(s) and the electrolyte bath is referred to as an electrolytic cell.

The electrolytic current, which flows in the electrolyte bath and the pad of liquid aluminum via the anodes and cathode components, brings about the aluminum reduction reaction and also makes it possible to maintain the electrolyte bath at a temperature of the order of around 950° C. The electrolytic cell is regularly supplied with alumina so as to compensate for the alumina consumption produced by the electrolytic reactions. Normally in production facilities, over 100 such cells or pots are connected in series.

An individual electrolytic cell can consist of a flat vessel with low sides made of steel plates. Inside this steel shell, there is generally a refractory layer which surrounds a carbon lining. A particular industrial example of a pot line consists of 288 GP 320 reduction cells producing aluminum using GAMI Technology. Electrolysis takes place in a cell called a pot or reduction cell which contains rodded carbon anodes, electrolyte called bath and carbon cathodes as the bed of the pot. A current of around 320 kA is passed through these pots which are connected in series to accomplish the electrolysis.

FIG. 1 shows an example of a prior art individual electrolytic cell. The cell is supported on a civil column (9) with insulating material (24) a steel rib (23). The pot shell assembly (22) contains calcium silicate bricks (21) and insulating bricks (20) with dry impervious material (19) with castable (18) and a cathode block (17). Other features of a typical cell are a manual ledge (16), side carbon blocks (15), SiC blocks (14), an anode assembly (12) and a carbon block anode (13) with an anode bus bar (10). A riser (11) generally extends out of the cell. On the top of the cell are an alumina hopper (1), and anode jack (2), a crust breaking device (3) and an anode clamp (4). The cell can have a side pot cover (5), an end pot cover (6) and a corner pot cover (7). Finally a pot shell (8) surrounds the pot.

During production the aluminum, a potline comprises rows of reduction electrolytic cells with the cells arranged transversely in each row, and with current being conducted through the bottom of the cells as shown in FIG. 2 which is a view of a prior art potline. A pot shell (25) can be seen with risers (28), a short circuit bus bar (27) and an anode bus bar (26). The flow of current (29) is shown with an arrow in FIG. 2.

Commonly the cells in an aluminum potline are arranged in rows as shown in FIG. 2. The distance between the rows, or rather the distance from the center line of one row to another is from 30 to 50 meters. Advantageously the cells are arranged in two or more, equal number of rows in which extra bus bars for the return current is avoided. The current in two neighboring rows flows in opposite directions.

Aluminum oxide (alumina) is the raw material from which aluminum is produced. The rodded anodes are hung on a bus bar by means of anode clamps and are dipped into the bath. Current is passed to the pot through these anodes. Alumina is continuously fed into the pot by a feeder system above the bath surface (1). As the alumina slowly dissolves, primary negative complex ions are formed in the bath. They eventually reach the anode surface and react forming carbon dioxide. Positive ions reach the cathode-forming aluminum metal. Aluminum metal being heavier than the bath settles down over the cathode. The metal can be tapped at regular intervals from the pot.

All the pots in the pot line are typically connected in series and carry around 320 kA of current which is passed through each pot to produce the aluminum metal. The bottom of the pot is made of carbon cathode blocks through which current passes into the embedded steel collector bar. From the collector bar, current passes through the aluminum bus bars to the next pot in the line. These aluminum bus bars are connected to the next pot raisers by a shorted joint.

From time to time, it is necessary to cut off the current to a pot for maintenance. The disadvantages of the prior art methods of pot cut off can be summarized as follows:

-   -   1. Intermittent load throw off, thus unnecessary inducement of         stresses in turbine & related equipment.

2. Boiler tube leakage—due to sudden change in the stresses. (Due to fast change of process parameters)

-   -   3. Turbine bearing vibrations.     -   4. Failure of gaskets for low pressure heaters.     -   5. Increase in specific coal consumption (as steam is getting         dumped into the condenser without doing any work).     -   6. Loss in generation.     -   7. Loss in PLF.     -   8. Loss in utilization of equipment.     -   9. High flue gas exit temperatures (Leading to carryover of hot         gases to the atmosphere.     -   10. Blackouts caused by the above problems.

Apart from the many disadvantages linked with pot room operation such as normal anode effect, increase in its duration happens because of outages. Of course one of the main disadvantages is simply the loss in production.

As per the GAMI design, a power outage is required every time pots are to be put off power. The following areas are affected due to power outages. According to the factors listed above, the main problems with cutting power to pots are:

-   1) Disturbances to the running operating pots. -   2) Impact on the power plant operation as well as on its equipment. -   3) Production losses due to decrease in line amperage during power     on of pots.

SUMMARY OF THE INVENTION

For the above and other purposes an object of the present invention is to provide for the continuous production of aluminum and to avoid production losses. For this purpose, a procedure to cut off a pot online has been developed i.e., without reducing the line amperage to 0 KA. Embodiments of the procedure of the present invention can be:

Leveling of the side beams of the reduction cell; punching holes on both sides of the pot to ensure that the bath does not overflow and to ensure the dipping of the anodes through these punch holes; blowing air thoroughly all around the insulated shorted joints of all the risers to clean the joints from dust and large chunks of bath; applying lubricating oil to the shorted joint bolts to conform proper movement of nuts; breaking the side crust of all the four corner anodes; tapping with at least one ladle of the bath; lowering of the anodes such that there is no liquid overflow over the deck plate onto the catwalk; leveling of the beams; lowering of beam by around 70 mm to 100 mm until the voltage falls below around 1.0 V-1.5 V at around 320 kA; loosening all the riser bolts of the shorted joint of all the five risers using pneumatic wrench after confirming that the anodes are dipped into the metal; and then taking the pot offline.

Prior to the cut out of the pot, bringing down the pot voltage to 1.5 V., and once the pot voltage is reduced, cut out operation is carried out in the normal procedure and the whole process is carried out at 320 kA.

As one object of the present invention there is provided a new procedure, which further provides increase in production, stability of operating pots and the elimination of frequent power outages.

In the following description, reference is made to the accompanying drawings which are shown by way of illustration to the specific embodiments in which the invention may be practiced. The following illustrated embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that other embodiments may be utilized, and that structural changes based on presently known structural and/or functional equivalents may be made without departing from the scope of the invention.

Given the following detailed description, it should become apparent to the person having ordinary skill in the art that the invention herein provides a novel engineered solution to the problem and a method permitting exploitation of significantly augmented efficiencies while mitigating problems of the prior art.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention:

FIG. 1 shows the front sectional view of a typical Aluminum Reduction Cell known in the art;

FIG. 2 shows the arrangement of pots in series during production process known in the art;

FIG. 3 shows the arrangement wherein three pots in series, Pot A & Pot C are online and Pot B is offline (cut off idle pot) in accordance with the present invention;

FIG. 4 shows the Shorted joint and short circuit bus bar of running pot in accordance with the present invention;

FIG. 5 shows the Shorted Joint in a non-running pot in accordance with the present invention;

FIG. 6 shows the position of the holes in the anode to ensure the dipping of the anodes through these punch holes.

Several illustrations and drawings have been presented to help explain the concepts and functioning of the present invention. The scope of the present invention is not limited to what is shown in the figures.

DETAILED DESCRIPTION OF THE INVENTION

A typical GP 320 Aluminum reduction cell known in the art is shown in FIG. 1. This pot shell assembly consists of pot super structure and pot shell. The pot super structure houses alumina hopper (1) for feeding alumina, an anode jack (2) for the anode bus bar movement, an anode bus bar (10), crust breaking device (3) to punch holes in the crust for alumina feeding and pot covers. Current flows from risers (11) to the anode bus bar (10). There are around 40 anodes clamped to the anode bus (10) bar with anode clamps (4) through which current flows through the bath.

The pot shell includes pot lining material like Calcium Silicate Bricks (21), Insulating Bricks ((20), Dry Impervious material (19), castable (18), SiC bricks (14) and side carbon bricks (15). Cathode blocks (17) are placed over the dry impervious material (19). The whole pot shell assembly is placed over civil columns (9) with proper insulation (24).

The earlier technology proposed by GAMI to a take a pot off line the pot line required the amperage to be reduced to 0 kA. This process of cutting out a pot generally takes around 30-35 minutes with 15-20 minutes at zero line current. This downtime impacts negatively on production, operation of the pots, and as well, on operation of the power plant.

FIG. 3 shows the arrangement wherein three pots in series, Pot A & Pot C are online and Pot B is offline (cut off idle pot) in accordance with the present invention. The anode bus bar (30), anode assembly (31), riser (32), pot shell (33), short circuit bus bar (34), cathode bus bar (35), civil column (36), insulating insert plates (37) and flow of current (38) can be seen.

FIGS. 4 and 5 show shorted joints. The riser (39), riser bolts (40), short circuit bus bar (41), insulation insert plate (42) and flow of current (43) can be seen. In the case of a normal running pot, shorted joints are insulated. Because of the insulation plates (42) current flows through the riser to the pot as shown by the arrows (43). In order to cut out a pot from the line, current from riser should flow from the riser to the shorted joint and not through the pot. To accomplish this, the insulation insert plates (42) are removed from between the riser (39) and the shorted bus bars (41) to make the current flow through the shorted joints. For the removal of the insulation plates, current is brought down to 0 kA and then the shorted joint bolts are loosened and a gap is made at the joint. After sufficient gap is made insulation plates are removed on both sides of each riser and then the bolts are tightened.

FIG. 5 shows the shorted Joint in a non-running (cut out) pot. Here current flow (43) can be seen leaving through the short circuit bus bar (41) rather than through the riser (39).

Under condition of failure of pot or for routine maintenance for any of the reasons as referred to above, the present invention discloses a procedure to take the pot offline (cut out) without reducing the line amperage to 0 kA. The principle used for the cut off is that the anode crust is being broken at the four corners of the pot. Then around 5-6 tons of liquid bath is tapped out. While tapping out liquid bath, the anodes are dipped into the metal pad bringing down the pot voltage to around 1.5 V. Once the pot voltage is reduced, the cut out operation is carried out in the normal procedure as mentioned above. In this case there is no need of lowering the current down to 0 kA; rather the whole process can be carried out at 320 kA. Returning to FIG. 3 an arrangement wherein three pots are in series can be seen. In FIG. 3, pot A & pot C are online while pot B is offline as discussed.

Prior to the cut out of the pot, some necessary preparations are required in the pot. As referred in FIG. 6, the beam level is maintained at around 150 mm with the A & B side beams are maintained in level with each other before cutting out the pot so that one can ensure later whether the beam levels are same or not. Thereafter at least six holes are punched on both sides (A & B) using PTM at the following points A2, A5, A8, B3, B6, and B9 illustrated in FIG. 6 so as to ensure the dipping of the anodes through these punch holes. Air Blowing can be done thoroughly all around the insulated shorted joints of all the risers to clean the joints of dust and large chunks of bath. Then lubricating oil can be applied on the shorted joint bolts and conform proper movement of nuts. The next job is usually to break the side crust of all the four corner anodes as shown in FIG. 6.

Then tapping of one ladle of bath is done, and if required, a second ladle is done depending on the cell voltage and bath level. As the bath is being tapped, lowering of the anodes into the metal is done so that no liquid can overflow over the deck plate onto the catwalk. The beams are held level during the lowering of the beam. Then Lowering of the beam is done gradually by a minimum of 70 mm to 100 mm until voltage falls below around 1.0 V-1.5 V at around 320 kA. Also a constant vigil should be made over the anodes so that they are dipped into the metal with a long crowbar through the six punched holes. All of the riser bolts of the shorted joint on all five risers are loosened using a pneumatic wrench. A gap can be introduced at the shorted joint of riser by means of short crow bar, and then the insulation insert plates can be removed. Insulated wooden wedges (called bamboos) can next be introduced between the joints so that they do not short with each other. After a thorough cleaning of the contact surface of the shorted joint with compressed air, the riser bolts can be tightened using the pneumatic wrench in the following preferred order: top left, bottom right, top right and bottom left. While this is the suggested order, any order is within the scope of the present invention. Finally the pot voltage is lowered to around 0.400 volts.

A main significance and advantage lies in the gain in production as the process involves no power reduction. This saves around 4.8 kWH per outage (per one pot cut out). It also reduces disturbances of normal operation of the pots and of the power plant.

Although the foregoing description of the present invention has been shown and described with reference to particular embodiments and applications thereof, it has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the particular embodiments and applications disclosed. It will be apparent to those having ordinary skill in the art that a number of changes, modifications, variations, or alterations to the invention as described herein may be made, none of which depart from the spirit or scope of the present invention. The particular embodiments and applications were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such changes, modifications, variations, and alterations are within the scope of the present invention. 

1. A process for online cut out of an aluminum reduction cell (pot) having a bath, liquid aluminum metal, side beams, insulated shorted riser joints and anodes comprising the steps of: leveling the side beams to around 150 mm. punching holes on both sides of the pot to ensure that the bath does not overflow and ensure the dipping of the anodes through these punch holes; breaking side crust from all corner anodes; blowing air thoroughly all around the insulated shorted joints of each riser to clean the insulated shorted joints of dust and chunks of bath; applying lubricating oil to joint bolts to conform proper movement of nuts; tapping with at least one ladle of the bath; Lowering of the anodes such that there is no liquid overflow; lowering of beams by around 70 mm to 100 mm until voltage falls below 1.0 V-1.5 V at around 320 kA; Loosening all riser bolts of the insulated shorted joints of all risers after confirming that the anodes are dipped into metal; and then removing insulating insert plates from the insulated shorted joints; Taking the pot offline.
 2. The process as claimed in claim 1, wherein at least six holes are punched on both sides of the pot.
 3. The process as claimed in claim 1, wherein the tapping is done using a second ladle.
 4. The process as claimed in claim 1, wherein 5-6 tons of liquid bath is tapped out ensuring the anodes are dipped into metal.
 5. The process as claimed in claim 1, wherein each of four corner anodes are broken so that the anodes have free movement during lowering of the anodes.
 6. The process as claimed in claim 1, wherein constant vigil is made over the anodes so that they are dipped into metal with a long crowbar through the punched holes.
 7. The process as claimed in claim 1 wherein a gap is introduced at the insulated shorted joint of a riser by means of a short crow bar so that insulated insert plates can be removed.
 8. The process as claimed in claim 1, wherein insulated wooden wedges are introduced between the insulated shorted joints so that the insulated shorted joints do not short with each other.
 9. The process as claimed in claim 1, wherein thorough cleaning of contact surfaces of the insulated shorted joint is performed.
 10. A method for taking an aluminum reduction cell offline without affecting operation of other reduction cells in series with said cell comprising the steps of: leveling side beams of said cell to around 150 mm; punching holes on both sides of said cell to prevent overflow; tapping a quantity of liquid bath from said cell; lowering anodes into metal in the said cell; lowering said beams by around 70 mm to 100 mm until voltage falls to between around 1.0 V. to 1.5 V.; loosening all riser bolts; removing insulating insert plates from insulated shorted anode risers; taking said cell offline.
 11. The method as claimed in claim 10 wherein a gap is introduced at the insulated shorted anode joint of a riser by means of a short crow bar so said insulated insert plates can be removed.
 12. The method as claimed in claim 10, wherein each of four corner anodes are broken so that the anodes have free movement during lowering of the anodes.
 13. The method as claimed in claim 10, wherein 5-6 tons of liquid bath is tapped out ensuring the anodes are dipped into said metal.
 14. The method as claimed in claim 10 wherein a final current of around 320 kA is maintained.
 15. The method as claimed in claim 10, wherein at least six holes are punched on both sides of the cell. 